This is a proposal to study excitation and conduction in heart muscle, using measurements of passive membrane properties, voltage clamp studies of membrane ionic currents, and computation of the role of various currents in electrogenesis. Most work is done on sheep Purkinje fibers, but rabbit ventricle and tissue cultured chick cells are also used. The voltage clamp technique is also used to study the relationship between voltage and tension and possible correlation with transmembrane calcium current. Some studies on the mechanism of action of cardiac drugs is planned.